The partitioning of dry matter to leaves was significantly higher than stems and roots except to stems at 30 DAS and roots at 45 DAS for the treatment RDF as shown in Chart 1. Afterwards, the dry matter partitioning to leaves and roots declined significantly whereas no such decline was observed for stems. Pods and nodules were not present at 30 DAS. The dry matter partitioning to nodules declined from 45 DAS to 60 DAS, while the partitioning of dry matter to pods was significantly higher at 60 DAS as compared to 45 DAS. Among the treatments, MR 63 and MH 17a received significantly higher dry matter in pods at 45 DAS while at 60 DAS, MR 54 and MH 8b2 showed higher dry matter in pods.
Droughted plants diverted significantly higher dry matter to roots and stems, while well watered plants to pods and grains. The dry matter mobilization to leaves was similar in both the environmental conditions
i.e. irrigated and stressed. Partitioning of dry matter to nodules was noticed highest in plants treated with
rhizobial strain MR63 at both the stages of observation (data not shown).
Leaf water potential decreased significantly under soil moisture stress as compared to irrigated control (-0.79 to -1.60 MPa). Decreased leaf water potential (yw) under stress may be due to loss of gradient (yw) between the soil and roots, which is the guiding principle for water movement and decline in transpiration pull (Table 1). Our results are in line with the findings of
Baroowa et al., (2015) who reported similar trends of water potential in mungbean under water deficit. Among the
rhizobial treatments, mean maximum leaf water potential was observed in MR 63 (-0.78 MPa) followed by MB 17a (-0.82 MPa) and it was minimum in RDF (-1.11 MPa). It can be inferred that maintenance of higher leaf water potential by the
rhizobial isolate MR 63 might be due to its better osmotic adjustment ability to maintain active photosynthesis and transpiration even under water stress conditions.
RWC is a reliable and widely used parameter of plant tolerance to dehydration. The results of relative water content showed reduction in RWC when exposed to drought stress (77.83 to 67.74%) irrespective of the
rhizobial treatments (Table 1). This might be due to decline in stomatal conductance and transpiration rate under drought stress. Such decline under drought stress coincides with the earlier findings in
Phaseolus vulgaris (Martinez
et al., 2007) and
Medicago truncatula (Nunes
et al., 2008). Plants treated with RDF + MR 63 showed higher RWC (75.7%) followed by RDF + MB 17a (74.9%) over the RDF control (69.2%) irrespective to the environment. The genotypic variations among rhizobial strains have shown varied behaviour under similar water conditions.
Photosynthetic rate was reduced significantly under the water stressed condition as compared to normal irrigated (12.53 to 8.75 µM m
-2s+). Similarly, transpiration rate and stomatal conductance also decreased significantly under the drought stress (4.45 to 2.10 mM m
-2s
-1 and 0.42 to 0.20 mM m
-2s
-1) respectively when compared on mean basis irrespective of the
rhizobial treatments (Table 1). Reduction in gas exchange parameters may be due to decreased leaf water potential and RWC under water stress which led to loss of leaf turgor and ultimately decreased stomatal conductance (Table 1). Similar result was reported in chickpea (
Cicer arietinum L.) by
Khadraji et al., (2017) and, in Common bean (
P. vulgaris L.) by
Kýymaz et al., (2019). As evident in Table 1, treatment RDF had lowest values of photosynthetic rate (8.51 µM m
-2s
-1) and transpiration rate (2.71 (8.51 mM m
-2s
-1). The gas exchange parameters get enhanced with the application of
rhizobial isolate MR 63 which has higher capability to combat drought than rest of the
rhizobial isolates. Therefore, this isolate showed maximum photosynthetic rate transpiration rate and stomatal conductance under stressed as well as under normal irrigated condition.
Drought caused a significant reduction in the yield (chart 2). Drought stress reduced the source strength by reducing photosynthesis and it may be due to adverse growing environment reflected in lower plant water status (Table 1). These findings were in line with the results of
Sharma and Dhanda (2014) and
Praharaj et al., (2016) who reported that seed yield of mungbean was affected by the irrigation amount and supplemental irrigation, particularly at the pod filling stage improve plant water status resulting increased yields. The present study revealed that the
rhizobial strain MR 63 produced the maximum yield under both the drought and irrigated environment (Chart 2). Similar response of
rhizobial inoculation induced yield improvement was reported by
Choudhary et al., (2019) and
Tena et al., (2016).
Association of seed yield with physiological traits
It is interesting to note that RWC showed strong relationships with photosynthetic rates, number of pods per plant and seed yield (Fig 1a, 1b and 2b).The higher the RWC, the higher were the rates of photosynthesis and number of pods per plant and vice-versa. Traits that showed good correlation with yield were rates of photosynthesis and RWC (Fig 2a and 2b).
